The plasma transferred wire arc (“PTWA”) process is a particularly useful high-pressure plasma coating process capable of producing high-quality metallic coatings for a variety of applications, such as the coating of engine cylinder bores. In the PTWA process, a high-pressure plasma is generated in a small region of space at the exit of a plasma torch. A continuously-fed metallic wire impinges upon this region wherein the wire is melted and atomized by the plasma. High-speed gas emerging from the plasma torch directs the molten metal toward the surface to be coated.
When utilizing the PTWA process, it is difficult to control the spray of the molten metal in industrial applications. Thus, one must protect against undesirable overspray of the molten metal on areas of the workpiece that extend beyond the treated surface. The current process for masking or protecting untreated surfaces on the workpiece include adding a coating spray to the untreated surfaces on the workpiece or by placing a dedicated or hard-tooled cover to fit over and protect the untreated surfaces. Spraying the unprotected surfaces with a coating is impractical on an industrial scale, as the coating must be applied and then removed after the PTWA process. Hard-tooled, dedicated covers are also not practical, as they limit the flexibility associated with automated tooling and machinery. In addition, neither of these processes provide for a highly accurate spray line or boundary between the treated surfaces and the non-treated surfaces of the workpiece.
It would be desirable to create a thermal metal spraying apparatus that properly masks sprayed metal from untreated surfaces of a workpiece without having to utilize supplemental coatings and/or tooling.